This invention relates to a method for detecting an object picture by an electron beam, and more particularly to such a method for detecting a pattern precisely and exactly at higher speed in, for example, the inspection of a mask use for integrated circuits (IC's).
To detect an object picture, the surface of an object is scanned by a well focussed electron beam and a signal varying in accordance with the condition of the object surface such as the presence or absence of a certain pattern is to be used. As shown in FIG. 1, when an object 2 is irradiated with an electron beam 1, there are generated signals such as reflected electrons 3, secondary electrons 4, absorption current 5 and X-rays 6, as is well known. In FIG. 1, arrows indicate the directions in which the electron beams and the electromagnetic waves travel.
The conventional technique using one of the above signals to investigate the structures of various objects is widely used in the field of scanning type electron microscopes. However, since some objects can only produce very low contrast, the conventional technique has frequently failed to generate a signal having a desired change in amplitude. Further, an electron beam having a large current density cannot be used since the destruction of the object surface by the electron beam must be prevented. Consequently, a signal having a large S/N (signal-to-noise) ratio cannot be obtained or a sufficient signal cannot be derived from each of the picture elements scanned at high speed. For example, in the case when a metalization pattern formed on a semiconductor substrate is detected by using reflecting electrons, secondary electrons or absorption current, it was revealed that a satisfactory S/N ratio could not be attained, which results in an adverse effect which restricts the speed in detecting patterns.